1. Field of the Invention
This invention is related to a motor speed detector and more particularly to a pulse pickup on a DC motor responsive to flux variations caused by movement of rotor teeth.
2. Description of the Prior Art
A great deal of prior art has been developed for detecting motor speed. A common approach that has been used for many years to obtain a pulse pickup indicative of speed is to connect an external gear to rotate with the motor shaft. Each time a gear tooth passes a proximity sensor, a pulse is generated. The sensor is often formed with a permanent magnet coupled through the gear to sensor windings. As the gear rotates, the reluctance of the magnetic flux path varies as a function of the position of the gear teeth generating a series of pulses indicative of motor speed. Exemplary of these types of digital sensors are U.S. Pat. Nos. 3,366,874; 3,688,306; 3,846,697; and 3,876,927. A problem with the sensors disclosed in these prior art patents is that it must be physically mounted to the rotor shaft of the motor whose speed is being sensed. Mounting the detector may present a problem since the motor shaft is generally driving some type of mechanical apparatus and room to mount the additional gear detector may not be available. A special motor having a take-off shaft at both ends, one of which is used for mounting the gear speed detector, may be required.
Some prior art motors include a speed detector mounted in a commmon housing. U.S. Pat. No. 3,790,832 which discloses an integral motor and tachometer is exemplary of such devices.
Various patents have disclosed apparatus for remote sensing of motor rotation. In some of these sensors, no modification of the motor is required. The speed sensor may be physically displaced from the motor housing. These sensors are normally effected by the motor magnetic field which is external to the motor. Exemplary of these type of apparatus are U.S. Pat. No. 3,188,620 and U.S. Pat. No. 3,404,339.
U.S. Pat. No. 3,466,477 discloses a motor speed sensing apparatus for use on an induction motor. In the apparatus disclosed in this patent, a pair of magnetic coil transducers are placed on opposite ends of the stator tooth portion. The induction machine rotor includes slots oriented at an angle with respect to the axis of the stator so that rotation of the rotor causes different induced currents in the coil transducers. The induced currents are combined to produce an output signal representative of the rotor speed.
U.S. Pat. No. 3,942,114 discloses a speed detector for a DC motor. The DC motor input voltage and a signal related to the input current are supplied to the speed detector system which is coupled substantially in parallel with the motor. The speed indicating system utilizes these inputs to generate a voltage proportional to the speed of the DC motor.
In many industries, such as food processing, there are a great number of small motors which must be synchronized. The degree of synchronism required, however, is not so great as to justify a really high precision positioning control. A problem with prior art speed detecting devices is that they are relatively expensive. Also, many of the prior art speed detectors require extensive modification of the motor and its electrical or mechanical connections. To be commercially viable for many industries, the pulse pickup must be very inexpensive. It is desirable to have a relatively inexpensive pulse pickup which is rugged, simple, easily installed and provides a digital signal related to motor speed.